This invention relates to an antenna unit and, more particular, to an antenna unit for use in a frequency band of a wireless Local Area Network (LAN).
In the manner which is well known in the art, the wireless LAN is an LAN using a transmission path except for a wired cable, such as electric waves, infrared rays, or the like.
Standardization of the wireless LAN is developed in IEEE (Institute of Electrical and Electronics Engineers) 802.11 Committee. That is, the IEEE 802.11 Committee develops specifications of the standard of the wireless LAN.
For example, IEEE 802.11a is a specification of a high-speed wireless LAN and a wireless access for 5 GHz band where the IEEE 802.11 Committee develops. A communication rate (a transfer rate) is about 20 Mbits/sec to 50 M bits/sec. A CSMA/CD (carrier sense multiple ac cess with collision detection) is used as an MAC (media access control). A modulation method of a physical layer is an OFDM (orthogonal frequency division multiplex).
On the other hand, IEEE 802.11b is a specification of the wireless LAN where the IEEE 802.11 Committee standardizes in September, 1999. The IEEE 802.11b uses frequencies of 2.4 GHz band and uses a direct spread (DS) as a modulation method. A transmission rate (a transfer rate) is 11 Mbits/sec or 5.5 Mbits/sec.
Furthermore, IEEE 802.11g is one of standards for the wireless LAN where the IEEE 802.11 Committee develops in June, 2003 and a specification for carrying out communications about 54 Mbits/sec at 2.4 GHz band. The OFDM is used as a modulation method. Accordingly, the IEEE 802.11g uses the frequencies of 2.4 GHz band which is similar to that of the IEEE 802.11b and supports the transfer rate of 54 Megabits/sec which is about five times of that of the IEEE 802.11b. In contrast to the IEEE 802.11a for supporting the transfer rate of 54 Mbits/sec, the IEEE 802.11g maintains compatibility with the IEEE 802.11b. In addition, although a maximum transfer rate of 54 Mbits/sec is similar to that of the IEEE 802.11a. the 2.4 GHz band is a “busy” frequency band where a lot of equipments except for the wireless LAN use. Therefore, it is said that a real transfer rate in the IEEE 802.11g becomes later than that of the IEEE 802.11a.
Inasmuch as the IEEE 802.11b and the IEEE 802.11g use the same use frequency band of 2.4 GHz band in the manner which is described above, both are collectively called IEEE 802.11b/g herein.
Various antenna devices used in the frequency band of the wireless LAN are already known in the art. By way of example, JP 2006-50517 A, which corresponds to U.S. Pat. No. 7,084,820 B2, discloses an antenna device having a resonance frequency of 2.456 GHz. This patent document will be called a first patent document. The first patent document discloses, in FIG. 8 thereof, the antenna device where an antenna pattern (a conductive pattern) is formed on a main surface (a front side, a first surface) of a substrate or a board. Specifically, the antenna device disclosed in the first patent document comprises, as the antenna pattern (the conductive pattern) formed on the main surface of the board, a first L-shaped wiring pattern, a second L-shaped wiring pattern, an L-shaped ground pattern, and an L-shaped signal pattern which are an U-shape as a whole. The first and the second L-shaped wiring patterns have first and second tips, respectively, which are opposed to each other with a space therebetween. The first and the second L-shaped wiring patterns are substantially symmetric with respect to a central line of the board. The L-shaped ground pattern has an end which is electrically connected to the second tip of the second L-shaped wiring pattern. The L-shaped ground pattern is formed on the main surface of the board so that it lies inside the second L-shaped wiring pattern with a space and along the second L-shaped wiring pattern. The L-shaped signal pattern has an end which is electrically connected to the first tip of the first L-shaped wiring pattern. The L-shaped signal pattern is formed on the main surface of the board such that it lies inside the L-shaped ground pattern with a space and along the L-shaped ground pattern. In addition, the antenna device further comprises a coaxial cable (feeder) which includes a central conductor and an outer conductor. At an end portion of the coaxial cable, the central conductor is electrically connected to another end of the L-shaped signal pattern and the outer conductor is electrically connected to another end of the L-shaped ground pattern.
At any rate, the first patent document discloses the antenna device which comprises the board having the first surface and the antenna pattern (a first transmission/reception pattern), formed on the first surface, for transmitting and receiving a radio signal (the radio wave) having a first frequency band.
On the other hand, JP 2005-79867 A discloses a flat-shaped antenna which has a sufficient input impedance and carries out efficient operation as the antenna although a metallic cabinet or a metallic part exists nearby. This patent document will be called a second patent document. The second patent document disclosed the flat-shaped antenna where favorable impedance matching is carried out at a frequency band of 2.4 GHz. More specifically, the flat-shaped antenna disclosed in the second patent document comprises a printed board made of a dielectric, a first antenna radiation element serving as one radiation element of a dipole antenna, a second antenna radiation element serving as another radiation element of the dipole antenna, a non-feeding element formed by a conductive pattern, a microstrip line serving as a feeding line for a radio wave, and a ground conductor for forming a ground side of the feeding line. The first antenna radiation element, the non-feeding element, and the microstrip line are formed on one surface (a front side, a first surface) of the printed board by a conductive pattern. The second antenna radiation element and the ground element are formed on another surface (a back side, a second surface) of the printed board by a conductive pattern. The dipole antenna is composed by feeding a high-frequency signal from an end portion of the microstrip line. The first antenna radiation element and the microstrip line are connected to each other around a central portion of the printed board. In addition, the non-feeding element may be omitted.
At any rate, the second patent document discloses the flat-shaped antenna (an antenna device) comprising a board having first and second surfaces (front and back sides) opposite to each other and the conductive pattern (a first transmission/reception pattern), formed on the first and the second surfaces, for transmitting and receiving the radio signal (the radio wave) having the first frequency bond.
In addition, JP 6-188610 A discloses a printed circuit board where patterns for antenna and a balun are formed on both sides or one side of the printed circuit board. This patent document will be called a third patent document. In the third patent document, a pattern for a dipole antenna and a coaxial guide fixing pattern for the balun are formed on a first surface (a frond side) of the printed circuit board while a branch conductive pattern is formed on a second surface (a back side) or the first surface of the printed circuit board. A central line of the coaxial guide is soldered to a feeding point of the dipole antenna while an outer conductor of the coaxial guide is soldered to the feeding point of the dipole antenna and the branch conductive pattern. The antenna and the balun disclosed in the third patent document are mounted inside a case of a portable radio equipment and use a frequency band between 421 MHz and 440 MHz.
At any rate, the third patent document discloses the printed circuit board (an antenna device) comprising a board having first and second surfaces (front and back sides) opposite to each other and the conductive pattern (a first transmission/reception pattern), formed on the first and the second surfaces (or the first surface) of the printed circuit board, for transmitting and receiving a radio signal (a radio wave) having a first frequency band.
Inasmuch as each of the antenna devices disclosed in the above-mentioned first through third patent documents is an antenna device where the antenna pattern is formed on the board, they are called board antennas.
In the manner which is described above, each of the antenna devices disclosed in the first through the third patent documents discloses only the antenna device (the board antenna) comprising the board and the antenna pattern (the first transmission/reception pattern) formed on one side (or both sides) of the board. The antenna devices can transmit and receive only the radio signal (the radio wave) having one kind of frequency band (the first frequency band).